Thermally Stable Hydrogel Optical Fibers Enable Soft Plasmonic Interfaces for Wearable and Implantable Monitoring

Hydrogel optical fibers (HOFs) offer soft, transparent, and biocompatible fiber platforms for biointegrated photonics, yet their practical use in wearable and implantable sensing is limited by thermally induced dehydration, shrinkage, refractive index drift, and optical loss. Here, we report a thermally stable HOF that couples hydrogen bond reinforcement with water-state regulation to stabilize both the hydrogel matrix and the fiber photonic interface. Polyacrylamide-based hydrogel fibers are fabricated by drawing-spinning, followed by thermal treatment, during which drawing-induced chain orientation, free-water evaporation, and glycerol-mediated hydrogen bonding generate dense hydrogen-bonded nanoclusters and a stable bound water. The resulting fibers exhibit smooth cylindrical morphology, a step-index-like refractive index profile, low optical attenuation of 0.136 ± 0.004 dB/cm, high flexibility, and stable light guiding from -10 to 80 °C. Benefiting from the thermally stable and metallizable hydrogel surface, a nanoscale gold film is integrated onto the fiber to construct a soft HOF surface plasmon resonance interface. The HOFs enable wearable skin temperature warning and implantable in vivo body temperature monitoring in mice. This work establishes a material-interface-device strategy for thermally robust hydrogel fiber photonics and expands HOFs from passive soft waveguides toward active plasmonic sensing platforms for wearable and implantable physiological monitoring.